1. Field of the Invention
The present invention broadly relates to a printer sheet feed device for conveying recording paper sheets, and a sheet feed control method thereof, and, more particularly, to a printer sheet feed device for conveying recording sheets with high precision, and a sheet feed control method thereof.
2. Description of the Related Art
A serial-type printer is often used, which prints one line while a print head moves along a platen, and then conveys the recording sheet by an amount corresponding to one line in order to perform printing on the next line.
FIG. 4 illustrates a thermal transfer printer as an example of a serial-type printer. In the thermal transfer printer, a flat platen 2 is disposed at about the center of a printer frame 1 such that the print face of the platen 2 is disposed substantially vertically. A carriage shaft 3 is disposed at a location of the frame 1 forwardly of and below the platen 2 and parallel to the platen 2. A flange-shaped guide section 4 is formed at the front edge of the frame 1, while a carriage 5 is mounted to the carriage shaft 3 and the guide section 4 such that it can reciprocate along the carriage shaft 3 and the guide section 4. A thermal head 6, functioning as a print head, is mounted to an end of the carriage 5 which opposes the platen 2, and can come into contact with and move away from the platen 2 by means of a drive mechanism (not shown). A ribbon cassette (not shown) is removably mounted to the upper face of the carriage 2 in order to guide an ink ribbon between the thermal head 6 and the platen 2. A take-up bobbin for taking up the ink ribbon and a supply bobbin are disposed at the upper face of the carriage 5, and engage the ribbon cassette take-up reel and the supply reel, respectively.
A sheet insertion opening 9 is formed behind the platen 2 in order to feed the recording sheet (not shown) forwardly of the platen 2. Sheet feed rollers 10 are disposed at the sheet insertion opening 9 in order to convey the recording sheet between the thermal head 6 and the platen 2. Press-contacting rollers 11, rotatably disposed below the sheet feed rollers 10, press-contact their sheet feed rollers 10. Actually, a plurality of the press-contacting rollers 11 are provided at different locations where they press-contact the sheet feed rollers 10 in the direction of the circumference, in which at the start of conveying one recording sheet, only the press-contacting roller at the upstream side in the direction of conveyance of the recording sheet contributes to the conveyance of the recording sheet, while at the end of the conveyance of the recording sheet, only the press-contacting at the downstream side in the direction of the conveyance of the recording sheet contributes to the conveyance of the recording sheet.
A sheet feed gear 12, mounted coaxially with the sheet feed roller 10, projects out from a side face of the frame 1. A motor gear 15 driven by a sheet feed motor 14 which is a stepping motor is connected to the sheet feed gear 12 via a plurality of transmission gears 13. Driving the sheet feed motor 14 rotationally drives the sheet feed roller 10 via the motor gear 15, the transmission gears 13, and the sheet feed gear 12, whereby a recording sheet, nipped between the sheet feed rollers 10 and the press-contacting rollers 11, is conveyed.
FIG. 5 is a view showing the critical portion of a printer sheet feed device, wherein the sheet feed gear 12 is mounted to one end of a rotating shaft 16 and coaxially with the sheet feed roller 10. The sheet feed roller 10 is mounted to the rotating shaft 16, and is disposed such that the outer peripheral surface of the rotatable press-contacting roller can press contact it. The sheet feed motor 14, being a stepping motor, is disposed in the vicinity of the sheet feed gear 12 in order to rotationally drive the sheet feed roller 10. A motor gear 15 is affixed to an output shaft 17 of the sheet feed motor 14, with the motor gear 15 and the sheet feed gear 12 connected via a first transmission gear 13a, a second transmission gear 13b, and a third transmission gear 13c, which are formed at the outer peripheral portion. The first transmission gear 13a comprises a large gear section 18a and a small gear section 19a formed coaxially therewith. The second transmission gear 13b comprises a large gear section 18b and a small gear section 19b formed coaxially therewith. The third transmission gear 13c comprises a large gear section 18c and a small gear section 19c formed coaxially therewith. The motor gear 15 engages with the large gear section 18a of the first transmission gear 13a, the small gear section 19a of the first transmission gear 13a engages with the large gear section 18b of the second transmission gear 13b, the small gear section 19b of the second transmission gear 13b engages with the large gear section 18c of the third transmission gear 13c, and the small gear section 19c of the third transmission gear 13c engages with the sheet feed gear 12. The rotation of the sheet feed motor 14 is slowed down by these gear groups in order to transmit the rotation to the sheet feed gear 12.
When the sheet feed roller 10 is rotationally driven by an amount corresponding to one line feed pitch, the gear ratio of the motor gear 15 and the transmission gears 13a, 13b, and 13c is set such that the motor gear 15 and the transmission gears 13a, 13b, and 13c always stop at the rotation start position. For example, the gear ratio of the motor gear 15, the transmission gears 13a, 13b, and 13c, and the sheet feed gear 12 is set at 3:4:4:5. With the gear ratio set thus, the motor gear 15 and the transmission gears 13a, 13b, and 13c stop at the same position both before the start of line feeding (shown in FIG. 6A) and after the line feeding (shown in FIG. 6B), so that the problem related to the eccentricities of each of the gears 15, 13a, 13b, and 13c does not occur.
More specifically, when the number of teeth of the motor gear 15 is 14, the number of teeth of the large gear section 18a and that of the small gear 19a of the first transmission gear 13a are 42 and 14, respectively; the number of teeth of the large gear section 18b and that of the small gear section 19b of the second transmission gear 13b are 56 and 14, respectively; and the number of teeth of the large gear section 18c and that of the small gear 19c of the third transmission gear 13c are 56 and 14, respectively; and the number of teeth of the sheet feed gear is 70.
When the desired printing is performed using the serial-type printer, the paper sheet is inserted into the sheet insertion opening and the sheet feed motor 14 is driven to rotate the sheet feed roller 10 via the motor gear 15, the transmission gears 13a, 13b, and 13c, and the sheet feed gear 12, whereby the recording sheet is conveyed such that its printing start position is at the printing position. Thereafter, with the thermal head 6 kept press-contacted with the platen 2, each of the thermal elements of the thermal head 6 are driven on the basis of a desired drive signal, while the carriage 5 is driven, as a result of which the desired printing is performed on the recording sheet.
Upon completion of one line of printing, while the thermal head 6 is kept separated from the platen, the sheet feed motor 14 is driven by a predetermined number of steps to rotate the sheet feed roller 10 by a predetermined angle, whereby the recording sheet is conveyed by one line feed pitch. Here, as described above, the motor gear 15, and the transmission gears 13a, 13b, and 13c rotate an integral number of times and returns to its initial start position, so that the problem related to eccentricities or the like of the gears 15, 13a, 13b, and 13c does not occur, resulting in a very accurate amount of sheet feeding.
In the above-described printer sheet feed device, however, at the moment the back edge of the recording sheet passes between the sheet feed roller and the press-contacting roller 11 disposed upstream in the direction of conveyance of the recording sheet, the recording sheet is bent due to the rigidity of the sheet so that a force which pushes out the recording sheet forwardly acts on the recording sheet (the mechanical condition of the sheet feed mechanism with respect to the recording sheet is changed), causing the recording sheet to be conveyed by an amount greater than the specified line feed pitch (the difference being equal to the amount of backlash between the sheet feed gear 15 and the transmission gear 13c), even when the press-contacting roller 11 disposed downstream in the direction of conveyance of the recording sheet is conveying the recording sheet by the correct amount. Therefore, when line feeding is terminated at a location immediately following this location, the excess amount of line feeding being in correspondence with the amount of backlash is not eliminated, so that a space is produced between lines during printing immediately following the termination of the line feeding, resulting in reduced printing quality.
Excessive line feeding tends to be particularly noticeable when a relatively rigid recording paper sheet, such as a relatively thick postcard, is used.
Other mechanical condition changes, such as displacement of the back edge of the recording paper sheet from the edge of the sheet guide section, occur, may change the line feed pitch.